Iodonium salts are important components of many imaging systems. They are useful for in-situ photochemical production of strong protic acids or free radical species which are subsequently used to initiate polymerizations or depolymerizations, or to react with an acid-sensitive functionality. Generally, they are thermally stable and photochemically labile, which is an ideal situation for imaging applications such as printing plates, lithographic films, and proofing systems, as well as for curing of, e.g., epoxide-type resins.
Symmetrical iodonium salts known in the art are commonly prepared by coupling an aromatic compound with iodate (e.g., potassium iodate), acetic anhydride, and sulfuric acid to yield a diaryliodonium bisulfate salt (See Crivello U.S. Pat. Nos. 3,981,897; 4,136,102; 4,151,175; 4,238,394; and 4,529,490 (assigned to General Electric); Beringer, F. M.; Drexler, E. M.; Gindler, E. M.; Lumpkin, C. C. J. Am. Chem. Soc. 1953, vol. 75, p. 2705; Beringer, F. M.; Falk, R. A.; Karniol, M.; Lillien, I.; Masullo, G.; Mausner, M.; Sommer, E. ibid 1959, vol. 81, p. 342; Klemm, E.; Alkahini, G.; Timpe, H. J. DD 290651; Crivello, J. V.; Lam, J. H. W.; Volante, C. N.; et al. J. Radiat. Curing 1977, vol. 4, p. 2; Crivello, J. V.; Lam, J. H. W. J. Polym. Sci. Symp. 1976, vol. 56, p. 383; and Crivello, J. V.; Lam, J. H. W. Macromolecules 1977, 10, 1307). Diaryliodonium bisulfate salts are generally too insoluble and unreactive to be of direct utility, and must be converted to other counter ions, typically by ion exchange procedures. A number of additional methods of preparing symmetrical and unsymmetrical diaryliodonium salts are known; most of them require the use of strong acids. (See Mason, I. Nature 1937, vol. 139, p. 150; Masson, I.; Race, E. J. Chem Soc. 1937, p. 1718; Masson, I.; Hanby, W. E. ibid. 1937, p. 1699; Masson, I. Argument, C. ibid. 1938, p. 1702; Beringer, F. M.; Bachofner, H. E.; Falk, R. A.; Leff, M. J. Amer. Chem. Soc. 1958, vol. 80, p. 4279; Collette, J. D.; McGreer, D.; Crawford, R.; Chubb, R.; Sandin, R. B. J. Amer. Chem. Soc. 1956, vol. 78, p. 3819; JP 63005040 Jan. 11, 1988; EP 119068; Crivello, J. V.; Lee, J. L. U.S. Pat. No. 4,399,071; Fukuyama, J. M.; Lee, J. L., Crivello, J. V. U.S. Pat. No. 4,992,571; Dektar, J. L.; Hacker, N. P. J. Org. Chem. 1990, vol. 55, p. 639; Research Disclosure RD 350042 1993; Crivello, J. V. U.S. Pat. No. 4,329,300 (May 11, 1982); Koser, G. F.; Wettach, R. H.; Smith, C. S. J. Org. Chem. 1980, vol. 45, p. 1543; Koser, G. F.; Wettach, R. H. U.S. Pat. No. 4,348,525 (Sep. 7, 1982); Koser, G. F.; Wettach, R. H. U.S. Pat. No. 4,826,635 (May 2, 1989); Crivello, J. V. U.S. Pat. No. 5,073,643 (Dec. 17, 1991); Crivello, J. V. U.S. Pat. No. 5,079,378 (Jan. 7, 1992); Kitamura, T.; Matsuyuki, J-i.; Nagata, K.; Furuki, R.; Taniguchi, H. Synthesis, 1992, vol. 10, p. 945; Dalziel, J. R.; Carter, H. A.; Aubke, F. Inorg. Chem 1976, vol. 15, p. 1247; Stang, P. J.; Zhdankin, V. V.; Tykwinski, R. Tetrahedron Lett. 1991, vol. 32, p. 7497; Stang, P. J.; Zhdankin, V. V.; Tykwinski, R. Tetrahadron Lett. 1992, vol. 33, p. 1419; Stang, P. J.; Tykwinski, R.; Zhdankin, V. V. J. Hetrocycl. Chem. 1992, vol. 29, p. 815; and U.S. Pat. No. 5,277,767. According to Miller, R. D. U.S. Pat. No. 4,786,441: "The prior art methods of preparation involve an exchange reaction between lithium triflate and the corresponding onium halide with the reaction being an equilibrium one taking place in an aqueous or mixed aqueous-organic medium. There is no force driving the equilibrium reaction in either direction and in general, the process is inefficient." When sulfuric acid is used, the previously mentioned bisulfate salts are formed.
Exchange of the bisulfate counter ion for a more useful counter ion, such as hexafluorophosphate, trifluoromethanesulfonate (hereinafter referred to as "triflate"), or p-toluenesulfonate can be effected by treatment of the bisulfate with an aqueous mixture of, e.g., the sodium or potassium salt of the corresponding desired acid. However, this exchange reaction can present difficulties if the desired salt is somewhat soluble in the aqueous reaction mixture. In these cases the exchange reaction cannot be forced to completion by precipitation of the exchanged salt, and an undesirable mixture of salts is obtained. It is known in the art to treat diaryliodonium bisulfate salts with aqueous sodium chloride to obtain the corresponding chloride salt, which is quite insoluble in the reaction mixture, thus precipitating out of solution and forcing completion of the exchange reaction. Exchange of the chloride counter-ion to a more useful counter-ion such as the triflate can be effected by treatment with the corresponding desired acid (or silylester). (See Dektar, J. L., Hacker N. P., J. Org. Chem., 1990, 55, 639; Research Disclosure RD 350042 1993; Beringer, F. M.; Drexler, E. M., Gindler, F. M., Lumpkin, C. C. J. Am. Chem. Soc., 1953, vol. 75, p. 2705) This exchange reaction is forced to completion by the elimination of hydrochloric acid (or trimethylsilyl chloride). This process works well to prepare the desired diaryliodonium salts but it requires isolation of the diaryliodonium chloride and an additional exchange reaction.
Thus, there has not been provided a convenient, simple and safe process for the preparation of useful diaryliodonium triflate salts.